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The AR-15 works on Stoner's direct gas Impingement system. When the bullet passes the gas port in the barrel the gas it tapped off to the bolt carrier and builds pressure and starts the rearward movement of the bolt carrier. The reason the sight (and the gas manifold) is a few inches back of the muzzle is that it takes that amount of "dwell time" to build the correct gas pressure before the bullet exits the muzzles so the bolt carrier will be moving at the correct speed to function the action. Too little dwell time for gas to build results in short cycles.
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